Bi-directional, dual speed film transport gear drive apparatus with an open loop control gear mesh routine

ABSTRACT

A dual speed film drive apparatus to achieve high speed and low speed film drive employs a limited speed range stepper motor with pinion input drive gear, and an output driven gear connected to a film advance mechanism. High and low speed drive is achieved by use of a pivoted intermediate gear drive module having high speed and low speed gear set positions to interconnect the stepper motor input drive gear to the output driven gear. Shifting of the gear module between gear set positions is accomplished by a DC motor operated cam drive mechanism which pivots the gear module selectively between the high speed and low speed gear set positions. The cam drive mechanism includes a camming slot having dwell segments at each end of the slot. An open loop control gear mesh procedure includes is also described for meshing the high speed and low speed intermediate gear drives with the input drive gear and output driven gear. In this procedure, the cam drive mechanism initially drives the gears toward mesh and then reverses the cam drive by a limited amount. If the input drive gear properly meshed with the intermediate gear drive, the dwell segment prevents the gear from being unmeshed. If stubbing occurred, the cam drive is still in the drive segment of the cam and backs the intermediate gear drive away while the stepper is activated by a partial tooth pitch. The cam drive is then driven forward again to engage the intermediate gear with the input gear. The stepper motor is then driven forward and back by equal amounts to clear any stub condition that might still exist between the intermediate gear and output driven gear interface. The open loop control routine avoids the need for complex and costly active feedback mechanisms to detect and clear a gear stubbing condition during gear shifting.

CROSS-REFERENCE TO RELATED APPLICATION

Cross-reference is hereby made to copending, commonly assigned U.S.application Ser. No. 09/162,902, entitled "BI-DIRECTIONAL, DUAL SPEEDFILM TRANSPORT GEAR DRIVE APPARATUS" filed concurrently herewith.

FIELD OF THE INVENTION

The present invention relates generally to the field of photographicfilm scanners and, in particular, to two-speed gear drive apparatus thatselectively operates film transport drive mechanisms at in forward andreverse directions at selectable, widely disparate speeds by selectivelymeshing one of plural intermediate gear drives with an input drive gearand an output driven gear.

BACKGROUND OF THE INVENTION

Film transport mechanisms used in photographic film scanners can beconfigured with DC or AC motors, stepper motors or servo motors used tooperate the transport mechanism at different film transport speeds. Filmscanners require precise positioning of the film strip, particularlyduring the image scanning operation for quality results. Drives for thispurpose typically include DC motors with position feedback using encoderwheels or disks. Transport speed can be changed by varying the voltagesupplied to the DC motor thus making the selection of appropriateoperating speed a relatively simple matter. However, the requirement forprecise positioning makes the feedback control with encoder wheels acomplex and relatively costly approach to transport design. Steppermotor drive systems have the advantage that precise positioning can bereadily achieved. However, a film scanner having a high/low speed ratioof, for example, on the order of 40 to 1, requires that a relativelycostly stepper motor be used to realize such a high range of speedvariation since inexpensive stepper motors are generally limited to alow range of speed variation. For film scanners intended to be massproduced for consumer applications, it is necessary to minimize theproduct cost which mitigates against use of the more costly steppermotor transport designs.

In the above cross referenced U.S. Application, a pivoted gear module isemployed to selectively engage a high speed or low speed intermediategear drive between an input drive gear attached to a low cost, limitedrange stepper motor, and an output driven gear attached to a film driveapparatus. The gear module is pivoted by means of a cam disk driven by areversible DC motor. When shifting gears from one intermediate drive toanother, the input load should be applied only when the teeth are inadequate mesh to prevent tooth damage. This requires that the gears areset to the correct center distance to ensure that the contact ratio isgreater than or equal to one, meaning that there is always at least onetooth of the driving gear in contact at the involute surface with onetooth of the driven gear at its involute surface. Alignment of the inputdrive gear and the output driven gear teeth to the teeth of theintermediate gears is important in order to swing the intermediate gearmodule fully into position. If the teeth on the input drive gear areoriented in such a way to the intermediate gear drive teeth that eithera tip to tip contact condition or two teeth on the input drive gearstraddling two teeth on the intermediate gear drive results, then thegear teeth will not be fully engaged. The same situation can occur atthe intermediate gear drive to output driven gear interface. Theseinterference conditions are referred to herein as "stubbing" of the gearteeth. If a stubbing condition at either or both of the gear interfacesduring the gear meshing procedure, then the intermediate gear drive willnot fully engage with the input drive gear and output driven gear andthe drive torque will not be properly transmitted to the driven outputgear. It is known to use active sensing and feedback mechanisms todetect the occurrence of stubbing and provide feedback to initiatecorrective actions to clear the stub condition. However, thesemechanisms can be complex and require added components and cost to thegear shift mechanism. There is therefore a need for an arrangement thatensures proper meshing of intermediate gear drives to input drive andoutput driven gears in a dual speed gear shift mechanism particularlyfor a film drive apparatus that does not require active feedbackmechanisms to detect and clear gear stub conditions and that maintainsaccurate relational position of the drive motor setting to the filmdrive positioning. The present invention satisfies that need.

SUMMARY OF THE INVENTION

In accordance with the invention, there is provided dual speed filmtransport gear drive apparatus that comprises a film drive stepper motorhaving an input drive gear; an output driven gear; a movable gear modulehaving first and second intermediate gear drives adapted for movementbetween first and second drive positions in which one of the first andsecond intermediate gear drives is selectively meshed with the inputdrive and output driven gears, the gear module having a cam follower;and a gear shift unit including a cam drive motor and a cam having a camslot engaged with the gear module cam follower, the cam slot havingextended dwell zones at each end of travel of the cam separated by agear shift drive zone. The dual speed gear drive apparatus also includesa controller which is operative, using open loop control, to causemovement of the gear module between the first and second positionsmeshing a selected one of the intermediate gear drives with the inputdrive gear and the output driven gear by performing the followingoperations:

(a) driving the cam drive motor forward for a first predetermined amountof time sufficient normally to drive the gear module to the selected oneof the positions with the cam follower in a dwell segment correspondingto the selected position;

(b) driving the cam drive motor in reverse to move the cam through alength of travel equivalent to a predetermined portion of the dwellsegment;

(c) driving the stepper motor through a fraction of a tooth pitch on theinput drive gear;

(d) driving the cam drive forward for a second predetermined amount oftime sufficient normally to drive the gear module to the selected one ofthe positions; and

(e) driving the stepper motor in a first direction for a plurality ofinput drive gear tooth pitches followed by driving the stepper motor inreverse for an equal number of input drive gear tooth pitches.

With this open loop control arrangement, any stubbing between the inputdrive gear and intermediate gear drive is first cleared by steps (a)-(d)and thereafter any stubbing between the intermediate gear drive and theoutput driven gear is cleared by step (e). If no stubbing of the gearshas occurred, the dwell segment of the cam drive prevents unmeshing ofthe input drive gear and intermediate gear drive during steps (b)through (d) above.

These and other aspects, objects, features and advantages of the presentinvention will be more clearly understood and appreciated from a reviewof the following detailed description of the preferred embodiments andappended claims, and by reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is an isometric exterior view of a film scanner embodying thepresent invention;

FIG. 2 is a perspective view of a film scanner chassis including filmtransport drive mechanism useful with the dual speed gear driveapparatus of the present invention;

FIG. 3 is a perspective view of portions of the film scanner showingfilm transport apparatus and an imaging housing assembly;

FIG. 4a is an isometric view of a dual speed, film transport gear driveapparatus of the present invention;

FIG. 4b is another isometric view of the gear drive of FIG. 4a with someof the structure omitted to better view the dual speed gear drivestructure employed;

FIG. 5 is an isometric view of the rear gear plate in the apparatus ofFIG. 4a showing the cam follower used in the shifting of the gear drivebetween high and low speed operation;

FIG. 6 is an exploded isometric view of the drive apparatus of FIG. 4a;

FIGS. 7 and 8 are end elevation views of the drive apparatus of FIG. 4useful in explaining the gear shifting operation of the apparatus;

FIGS. 9a and 9b are diagrammatic illustrations of gear features usefulin explaining the gear-meshing operation of the drive apparatus of FIG.4;

FIG. 10 is a schematic illustration of a cam drive disc used in thecontrolled shifting of gears in the gear drive apparatus of FIG. 4a; and

FIG. 11 is a program flow chart for a gear mesh procedure used in thegear drive apparatus of FIG. 4a.

DETAILED DESCRIPTION

In FIG. 1, there is shown a compact film scanner 10 adapted for use withAPS film cartridges to convert film images to digital images. Theillustrated scanner comprises an outer casing 12 and a front bezel 14which enclose an internal film drive chassis to be described in moredetail below. The chassis is provided with an APS film cartridge chamber16 for receiving a manually inserted APS film cartridge. A hinged,spring-loaded door 18 includes a locating device 20 which acts to engagethe end of the inserted film cartridge (not shown) as the door is closedto aid in properly positioning the film cartridge in the chamber 16. Alatch hook 22 on the door engages an internal latching mechanism 24 tolock the door in the closed position. When it is desired to remove thefilm cartridge, a door release button 26 is actuated to unlock the latchmechanism allowing the door to spring open. An internal spring loadedmechanism (not shown) within the cartridge chamber forces the cartridgepartially outward, allowing the user to grasp and remove the cartridge.

Referring to FIG. 2, a general description of the internal film drivechassis 30 of the scanner 10 is presented here. The illustrated chassiscomprises a lower frame 32 and an upper plate 34 which, when assembledas shown, define the film cartridge chamber 16, a film takeup chamber 36and a film path 38 longitudinally extending between the two chambers. Afilm transport drive roller mechanism 40, including nip roller sets 41and 42, is located near the film cartridge chamber 16. A pair ofelongated apertures 39a, 39b are provided in upper plate 34 between thenip roller sets and the takeup chamber to allow access by magneticread/write heads (not shown) to magnetic data tracks on the magneticrecording layer formed on the APS film as the film is driven along thefilm path, thereby enabling data transfer to and from the film magneticlayer, in known manner, as part of the film scanning process. Lowerframe 32 is provided with depending skirt walls 37 that form a cavity inwhich is mounted imaging apparatus 60.

Referring to FIG. 3, film transport drive apparatus 40 and imagingapparatus 60 are shown isolated from the surrounding chassis elements 32and 34 for ease of visualization and description. One end of the imagingapparatus 60 comprises a film scanner end 62 which is snap mounted intoa film scanner opening formed in the chassis 30. An elongated lightsource (not shown) is mounted on retention arms 63 to shine lightthrough film in the scanner end 62 and into an optical light pathinternal to the imaging apparatus 60. The opposite end of imagingapparatus 60 comprises an image sensor housing 64 in which a linear CCDimage sensor (not shown) is mounted. The film scanner end 62 isstraddled by film transport drive apparatus 40 comprising film drive niproller sets 41 and 42. Nip roller set 41 comprises main drive rollers43a, 43b mounted on main drive shaft 44 and pressure rollers 47a, 47bmounted on shaft 49a. Nip roller set 42 comprises slave drive rollers45a, 45b mounted on slave drive shaft 46 and pressure rollers 47c-47dare mounted on shaft 49b. The pressure rollers are held in compressionagainst the drive rollers by means of pressure clamps 50 and 51 andtension springs 52 and 53, respectively. The main drive rollers aredrivingly coupled to the slave drive rollers by means of main drivepulley 55, elastic drive belt 56 and slave drive pulley 57. Gear 54 ispressed on the end of main drive shaft 44 and comprises an output drivengear forming part of the dual speed gear drive apparatus describedherein. The imaging apparatus 60 is described more fully in copending,commonly assigned U.S. patent application Ser. No. 09/83,604, filed May22, 1998, entitled "Imaging Apparatus for a Photographic Film ImageScanner", the disclosure of which is incorporated herein by reference.The film transport drive apparatus 40 is described more fully incopending, commonly assigned U.S. patent application Ser. No.09/085,730, filed May 27, 1998, entitled "Film Drive Apparatus for aPhotographic Film Scanner", the disclosure of which is incorporatedherein by reference.

Turning now jointly to FIGS.4-6, there will now be described thebi-directional, dual speed gear drive apparatus 70. This gear drive isused to drive the film transport drive apparatus 40 in forward andreverse directions in a selectable one of a high or low film transportspeed of widely disparate speed settings. Drive apparatus 70 comprises asupport frame 80 having a rear mounting plate 82 and a pair of frontmounting plates 88 and 89. Rear mounting plate 82 supports a filmtransport drive motor unit 69 including a stepper motor 72, an optionalshield 75 and cooling fan 73. Stepper motor 72 is reversible for drivingthe film transport mechanism 40 selectively in either a forward orreverse film transport direction. Stepper motor 72 is preferably a lowcost motor having a limited step range of, for example, 200 to 600pulses per second. The shaft of stepper motor 72 extends through anopening in the rear mounting plate 82 and has a pinion 71 mountedthereon, the pinion 71 comprising an input drive gear of the gear driveapparatus 70. Gear 54 mounted on main roller drive shaft 44 of the filmtransport apparatus 40 comprises an output driven gear of the gear driveapparatus 70. The input drive gear and output driven gear are mounted infixed coaxial orientation.

The gear drive apparatus 70 further comprises a movable gear module 78having a rear gear plate 100 and a forward gear plate 102. The gearplates are held in spaced apart relationship by stanchions 103 pressedinto mounting bosses 104. A pair of intermediate gear drives 84 and 86are rotatably supported between the gear plates. Gear drive 84, as bestseen in FIG. 4b, is an integral shaft with an input gear 84a and anoutput gear 84b having a low speed reduction ratio of, for example 4:1,and serves as a high speed intermediate gear drive 84. Gear drive 86,comprises a cluster of three compound, speed reduction gears 107, 108and 109 having a relatively high speed reduction ratio, for example150:1, and serves as the low speed intermediate gear drive. The gearmodule is pivotably mounted on post 110 cantilevered from rear mountingplate 82. To this end, rear gear plate 100 is provided with a spacercollar 106 which slips over the post 110. The forward gear plate issupported and held freely in place on snap fingers 111 formed on thefree end of post 110. Rear gear plate 100 additionally includes anelongated aperture 120 to provide clearance for input drive gear 71 asthe gear module is pivoted.

The gear drive apparatus 70 further includes a gear shifting mechanismcomprising a gear shifting cam disc 76 driven by the reversible DC motor74 via a pinion gear 77 extending through the rear mounting plate 82 andengaging gear teeth formed on the perimeter of the cam disc. The face ofthe cam disc has a generally spiral cam slot 79 which engages camfollower pin 100a formed on the back side of rear gear plate 100. Thespacer collar 106 on gear plate 100 establishes the necessary spacebetween the rear gear plate and the rear mounting plate 82 to allowengagement of the cam disc with the cam follower pin. Stepper motor 72and DC motor 74 are controlled by a separate drive controller 130 in amanner described in detail below.

In operation, with reference first to FIG. 7, when it is desired tooperate the film transport drive in the high speed mode, DC motor 74 isactuated by controller 130 to drive the cam disc 76 in thecounterclockwise direction as shown by arrow A in FIG. 7 thereby drivingthe cam follower pin 100a on the rear side of gear plate 100 toward theouter circumference of the disc 76 which pivots the gear module 78 inthe clockwise direction as shown by arrow B. This motion of the gearmodule disengages the low speed intermediate gear drive 86 from theinput drive gear 71 and output driven gear 54, shown in dotted outlineand engages the high speed intermediate gear drive 84 with the inputdrive gear 71 and the output driven gear 54. Similarly, with referencenow to FIG. 8, when it is desired to operate the film transport in theslow speed mode during image scanning, DC motor 74 is actuated in thereverse direction to drive the cam disc 76 in the clockwise direction asshown by arrow C. The camming action of the cam disc on cam follower pin100a drives the follower inward on the disc. This action pivots the gearmodule 78 in the counterclockwise direction shown by arrow D whichdisengages the high speed gear drive 84 from the input and output gears71 and 54 and engages gears 107 and 109a of the low speed intermediategear drive 86 with the input and output gears 71 and 54, respectively.

With the low speed intermediate gear drive 86 meshed with the input andoutput gears, film transport speeds in the range of 1 mm/second can beachieved. With a stepper motor step range of 200-600 pulses per second,a variety of scan resolutions can be achieved in this speed range. Withthe high speed intermediate gear drive engaged between the input andoutput gears, transport speeds in the range of 100 mm/second areachieved. Thus a significant shift in film transport speeds is achievedwith a low cost stepper motor operating in a relatively narrow range ofstep rates.

It will be appreciated that alignment of the teeth of the input drivegear 71 and the output driven gear 54 to the teeth of the selectedintermediate gears is important in order to swing the intermediate gearmodule fully into position. If the teeth on the input drive gear areoriented in such a way to the intermediate gear teeth that two teeth onthe input drive gear 71 straddle two teeth on the intermediate gear IG,as shown in FIG. 9a, or a tip to tip contact condition exists, as shownin FIG. 9b, then the gear teeth will not be fully engaged. Theseconditions are referred to herein as "stubbing". The same situation canoccur at the intermediate to output driven gear interface. If either ofthese conditions occur or if both occur simultaneously, then theintermediate gear module 58 will not fully engage with the pinion andoutput gear and the drive torque from the stepper motor 72 will not beproperly transmitted to the driven output gear 54.

There will now be described a mechanism and procedure in accordance withthe present invention by which proper meshing is ensured before thestepper motor drive is actuated to advance film in the scanner at eitherhigh or low speed. Referring to FIG. 10, the cam slot 79 of the gearshift cam 76 has a cam profile that includes three main segments: a lowspeed segment S₁ with a constant radius r₁ for approximately 180°, aconstantly increasing radius segment D, which moves the gear modulebetween the low speed and the high speed positions, and a high speedposition with constant radius r₂ for approximately 180°. The function ofthe constant radius segments is to set the appropriate gear centerdistance the intermediate gear drive and both the input drive gear andoutput driven at a distance produces a contact ratio between the teethgreater than or equal to one in accordance known practice (meaning theinvolute surface of the gear teeth are always in contact with each otheras the gears rotate). The 180 degrees of dwell at these positions allowsfor substantial variability in cam rotation during the gear meshprocedure without affecting gear engagement. The amount of dwell alsoallows the cam to be rotated back and forth less than the amount ofdwell without moving the gears from the optimum meshed position. Aprogrammable controller 140 (FIG. 4a) is coupled to the DC motor 74 andthe stepper motor 72 and is programmed to perform open loop gear shiftcontrol procedure as will now be described.

The flow chart of FIG. 11 illustrates the basic gear shift procedure.The shift routine starts at step 200. At step 202, half voltage (2.5 Vout of a possible 5.0 V) is applied to the DC motor which drives the camfrom the low speed to high speed or high speed to low speed positiondepending on the polarity of the applied voltage. Step 204 maintainspower to the DC motor for a fixed amount of time, longer thancharacteristic time the shift system takes to fully shift from one tothe other intermediate gear drive. If the teeth of all three gears, thatis, the input drive gear, output driven gear and intermediate geardrive, are aligned properly then the cam 76 fully rotates and the camfollower pin 100a on the gear module 100 is at the corresponding end ofthe cam slot 79. If the teeth of the input drive gear or the outputdriven gear are stubbed on the intermediate gear drive as describedabove, then the cam does not fully rotate and stops with the camfollower pin 100a lodged in the drive segment D, short of thecorresponding constant radius dwell segment S₁ or S₂ of the cam slot 79.It should be recalled that, because this is an open loop control, themesh condition of the gears is not known at this stage.

Steps 206 and 208 apply full voltage (5.0 V) to the DC motor in theopposite direction for a time long enough to rotate the cam more than 90degrees but less than the total dwell angle of 180 degrees. If the shiftin steps 202 and 204 was successful and the cam 76 had fully rotated,then this movement within the dwell segment does not change theengagement of the gear teeth and the teeth stay in full mesh. If theteeth had stubbed and the cam had not fully rotated then this movementbacks the gear teeth away from each other since the cam follower pin100a was lodged in the drive segment D at the conclusion of step 204.

At step 210, the stepper motor is actuated to move the input drive gearteeth slightly less than one half the tooth pitch of the input drivegear. This realigns the input drive gear teeth with the matingintermediate gear drive teeth. If the initial shift had fully rotatedthe cam the movement in steps 203 and 204, the teeth remained in meshduring the reversal of the DC motor and this latter of the stepper motorinput drive merely nudges the intermediate gear. If the initial movementresulted in a tooth stub and the second step backed the gears away fromeach other then the stepper motor movement realigns the teeth such thatthey are not in an orientation that causes stubbing of the intermediategear teeth with the input drive gear teeth. At step 212, half voltage(2.5 V) is applied to the DC motor in the forward direction to drive thecam toward the fully rotated position. This half voltage is maintainedduring the next step 214. If the only stubbing condition that hadexisted was between the input drive gear and the intermediate geardrive, steps 206-210 would have cleared the condition and step 212 wouldthen result in full meshing of the intermediate gear drive with both theinput drive gear and the output driven gear. However, if stubbing of theintermediate drive gear with the output driven gear had also existed andwas not incidentally cleared during the course of steps 206-212, thenthe output side stub still exists at the conclusion of step 212 and thecam may not fully rotate. It may be noted at this point that stubbing onthe output side is something of an unstable condition.

In the next step 214, the stepper motor is actuated in one direction fora fixed number of steps and then reversed for the same number of steps.During this move, if the stub at the output gear interface still exists,the input drive gear to intermediate gear mesh is enough to turn theintermediate gear, thereby driving the stub condition off. At thisinstant, since power is still being supplied to the DC motor, the camthen rotates to drive the gear module fully into position. Preferablythe amount of stepper motor rotation in each direction is at least onefull revolution and more preferably is two full revolutions. This equalamount of rotation in each direction returns the position of the motorto where it was at the beginning and maintains the proper alignment ofthe motor step count to the film position in the film gate of thescanner. The equal movements of the stepper motor result in negligiblechange in the position of the output gear if the initial shift wassuccessful and a tolerable change if the last stepper movement clearedthe output gear tooth stub. Step 216 then removes power from the DCmotor driving the cam and the gear shift routine ends at step 218.

The advantages of the gear mesh procedure just described are that thetwo speed gear mechanism can operate in "open loop" manner thus avoidingthe need for an additional position sensor and related electroniccircuitry to detect the occurrence of a gear stubbing condition; andalso it ensures that the intermediate gear mechanism is always fullyengaged with the stepper motor input drive gear and the output drivengear thereby minimizing the chance of gear damage during operation.

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations and modifications can be effected within the spirit and scopeof the invention.

What is claimed is:
 1. A method of operating a dual speed film transportgear drive apparatus of the type having:film drive stepper motor havingan input drive gear; an output driven gear; a movable gear module havingfirst and second intermediate gear drives adapted for movement betweenfirst and second drive positions in which one of said first and secondintermediate gear drives is selectively meshed with said input drive andoutput driven gears, the gear module having a cam follower; a gear shiftunit including a cam drive motor and a cam having a cam slot engagedwith said gear module cam follower, the cam slot having extended dwellzones at each end of travel of the cam separated by a gear shift drivezone; and a controller operative, using open loop control, to causemovement of said gear module between said first and second positionsmeshing a selected one of said intermediate gear drives with said inputdrive gear and said output driven gear by performing the following stepscomprising:(a) driving said cam drive motor forward for a firstpredetermined amount of time sufficient to drive said gear modulenormally to the selected one of said positions with said cam follower ina dwell segment corresponding to said selected position; (b) drivingsaid cam drive motor in reverse to move said cam through a length oftravel equivalent to a predetermined portion of said dwell segment; (c)driving said stepper motor through a fraction of a tooth pitch on saidinput drive gear; and (d) driving said cam drive forward for a secondpredetermined amount of time sufficient normally to drive said gearmodule to said selected one of said positions; whereby any stubbingbetween said input drive gear and intermediate gear drive is cleared bysteps (a)-(d) and, if no stubbing of said gears has occurred, said dwellsegment of said cam drive prevents unmeshing of said input drive gearand intermediate gear drive during step (b).
 2. The method of claim 1wherein said controller is operative after step (d) to perform theadditional step of:(e) driving said stepper motor in a first directionfor a predetermined number of motor steps followed by driving saidstepper motor in reverse direction for an equal number of motor steps;whereby any stubbing between said intermediate gear drive and saidoutput driven gear is not cleared by steps (a)-(d) is cleared by step(e).
 3. The method of claim 1 wherein said gear module is pivotablymounted, said cam comprises a rotatable disk having a face surface andsaid cam slot is formed in said face surface generally in the shape of aspiral with dwell segments at each end of the slot, each dwell segmenthaving a predetermined length of constant radius.
 4. The method of claim3 wherein said dwell segments are each approximately 180° ofcircumferential length.
 5. The method of claim 3 wherein in step (b)said predetermined portion of said dwell segment is about 90°.
 6. Themethod of claim 2 wherein in step (e) said stepper motor is driven by anequal amount of rotation in each of said first and second directions,said amount of rotation being at least one revolution.
 7. The method ofclaim 6 wherein said amount of rotation is about two revolutions.